The present invention relates generally to a method and apparatus for removing contaminants dissolved in groundwater and, more particularly, to in situ anaerobic reactive zones for removing contaminants dissolved in groundwater.
Contamination of groundwater with potentially hazardous materials is a common problem facing industry, the government, and the general public. Frequently, as a result of spills, leakage from storage facilities, or surface discharges, contaminants percolate into groundwater, thereby posing a threat to drinking water supplies. While groundwater is not as susceptible to pollution as surface water, once polluted, its restoration is difficult and long term. Various methods for withdrawing and treating contaminated groundwater have met with limited success. Typically, groundwater is removed from the saturated zone, treated, and then returned to the saturated zone. These known methods involve great expense and incur risks inherent in treating heavy metals and other contaminants, such as nitrates, present in the sub-surface.
Preferred embodiments of the method and apparatus of the present invention utilize the principle of in situ anaerobic reactive zones for the precipitation of metals, microbial denitrification, reductive dechlorination, and/or the precipitation of chromium. Precipitation is a process of producing a separable solid phase within a liquid medium. The method may involve installing injection wells into the saturated zone of contaminated soil. A substantially impervious well casing may be placed in the borehole with a fluid-permeable section at its base.
In one preferred method of the present invention, a sufficient amount of carbohydrates and sulfates may be metered into the conduit under pressure to facilitate proper mixing and dispersion in the saturated zone to achieve desired sulfate reducing and methanogenic conditions. The present invention may utilize molasses extract to introduce both carbohydrates and sulfates instead of injecting carbohydrates and sulfates separately via two injection streams. A mixing pump at the base of the conduit may be utilized to provide a more homogeneous mixture within the conduit. The mixture may then permeate through the fluid-permeable screen of the conduit at its base and mix with the surrounding groundwater.
Heterotrophic, sulfate reducing, and denitrifying microorganisms indigenous to the soil microflora may then serve as catalysts for the precipitation of metals, microbial denitrification, reductive dechlorination, and the precipitation of chromium. Two reactions involving microbes may form part of some methods of the present invention. One reaction utilizes the carbohydrates and the dissolved oxygen in the groundwater to form carbon dioxide and water. The result of this reaction causes a depletion in the oxygen level and leads to the formation of substantially anaerobic conditions in the saturated zone. In particular, this reaction preferably reduces the level of dissolved oxygen in the zone being treated to less than about 0.5 mg/l. Furthermore, this reaction preferably creates biogeochemical conditions in the zone being treated that include a redox potential of less than about -250 mv and a dissolved organic carbon to contaminant ratio of greater than about 50:1.
Under the sulfate reducing and methanogenic conditions, the sulfates present may be reduced to form sulfide ions. The sulfide ions may then react with the dissolved heavy metals to form a solid precipitate which eventually is filtered out by the soil matrix. There is no need to remove the precipitate from the soil matrix because it is insoluble and harmless.
As an example, the following reactions are indicative of the process utilizing sugar with sulfate to precipitate dissolved lead, zinc, mercury, and nickel from groundwater: ##EQU1##
In addition to the precipitation of metals, the aforementioned sulfate reducing and methanogenic conditions caused by the injection of carbohydrates and sulfates lead to the reduction of chlorinated hydrocarbons. Chlorinated hydrocarbons include tetrachloroethylene (PCE), trichloroethylene (TCE), dichloroethylene (DCE), and vinyl chloride (VC). Each of these chlorinated hydrocarbons can contaminate the groundwater supply. For example, exposure to high levels of PCE and TCE may cause harm to the nervous system, the liver, the kidney, or even death, and VC may be a common and highly carcinogenic groundwater contaminant.
As shown below, the injection of carbohydrates and sulfates into the saturated zone leads to the reduction of PCE to TCE to DCE to VC and eventually to ethene, a harmless, inert gas. The ethene may eventually be stripped into the soil gas. As a result, a preferred method of the present invention removes dissolved PCE, TCE, DCE, and VC contamination from the groundwater. ##STR1##
In addition, the injection of carbohydrates and sulfates to achieve the aforementioned biogeochemical conditions in the saturated zone leads to the precipitation of chromium, another harmful contaminant. In particular, the injection of carbohydrates and sulfates leads to the reduction of hexavalent chromium to trivalent chromium. Trivalent chromium is then precipitated as chromic hydroxide. EQU Cr.sup.6+ .fwdarw.Cr.sup.3+ EQU Cr.sup.3+ +3OH.fwdarw.Cr(OH).sub.3
The use of molasses extract to achieve each of the above reactions is a unique application. The precipitation of metals and reductive dechlorination in an in situ reactive zone rather than in an above ground aqueous phase reactor is also a unique development. The hydrogeological manipulations used in this invention to cause a homogeneous in situ reactive zone in all three dimensions is also unique. Using the soil matrix itself to filter out the insoluble metal precipitates and the inert gas is also unique.
The in situ reactive zone concept can also be applied to microbially denitrify the dissolved nitrates (NO.sub.3.sup.-) to nitrogen gas. The technologies used today to decontaminate dissolved NO.sub.3.sup.- in groundwater involve pumping the contaminated groundwater and using above ground technologies, such as ion exchange beds, reverse osmosis, or anaerobic bioreactors. In this invention, the injection of carbohydrates will create an anaerobic zone due to the depletion of the dissolved oxygen. The carbohydrates may be injected in the form of molasses. In the saturated zone, the denitrifying microbial consortia will degrade the NO.sub.3.sup.- ion first to a nitrite ion (NO.sub.2.sup.-) and eventually nitrogen (N.sub.2) gas. The nitrogen gas, thus formed, will be eventually stripped into the soil gas. As a result, the dissolved nitrate contamination may be removed from the groundwater. EQU C.sub.6 H.sub.12 O.sub.6 +6O.sub.2 .fwdarw.6CO.sub.2 +6H.sub.2 O EQU NO.sub.3.sup.- .fwdarw.NO.sub.2.sup.- .fwdarw.N.sub.2 .uparw.
The present invention may be practiced utilizing single injection wells or in multiple clusters depending upon the depth of the saturated zone, the geology of the remediation site, and the degree of mixing that may be created by each individual injection well. It should be recognized that required microbial cultures may be added to the soil matrix. This may be required where the indigenous microbes are not present in sufficient numbers to initiate the reactions.
In addition to the novel features and advantages mentioned above, other objects and advantages of the present invention will be readily apparent from the following descriptions of the drawings and preferred embodiments.